Sunday, December 11, 2016

Restoration of S. osborni Illustration by L. Xing and Y. Liu.

Saurolophus (meaning “lizard crest”) is a genus of large saurolophine hadrosaurid dinosaurs that lived about 70.0–68.5 million years ago, in the Late Cretaceous of North America and Asia; it is one of the few genera of dinosaurs known from multiple continents. It is distinguished by a spike-like crest which projects up and back from the skull. Saurolophus was a herbivorous dinosaur which could move about either bipedally or quadrupedally.

The size of the two Saurolophus species compared to a human

The type species, S. osborni, was described by Barnum Brown in 1912 from Canadian fossils. A second valid species, S. angustirostris, is represented by numerous specimens from Mongolia, and was described by Anatoly Konstantinovich Rozhdestvensky.

S. angustirostris skeleton and skull

Saurolophus is known from material including nearly complete skeletons, giving researchers a clear picture of its bony anatomy. S. osborni, the rarer Albertan species, was around 9.8 m (32 ft) long, with its skull 1.0 m (3.3 ft) long. Its weight is estimated at 1.9 tonnes (2.1 tons). S. angustirostris, the Mongolian species, was larger; the type skeleton is roughly 12 m (39 ft) long, and larger remains are reported. Aside from size, the two species are virtually identical, with differentiation hindered by lack of study.

The most distinctive feature of Saurolophus is its cranial crest, which is present in young individuals, but is smaller. It is long and spike-like and projects upward and backward at about a 45° angle, starting from over the eyes. This crest is often described as solid, but appears to be solid only at the point, with internal chambers that may have had a respiratory and/or heat-regulation function.

Skulls in Moscow Paleontological Museum. Author: Pavel Bochkov

The holotype of S. angustirostris is a skull and postcrania, so the cranium of the species is well-described. Bell et al. re-evaluated the entire species in a 2011 publication with Acta Palaeontologica Polonica. Their description found the skull to be generalized among hadrosaurines, and are much larger than any skulls of S. osborni. The most unusual feature for a hadrosaurine is the long, protruding, solid crest that extends upwards diagonally from the back of the skull roof. Unlike lambeosaurines, the crests are made up completely of the nasal bone. The premaxilla bones make up almost 50% of the entire skull length, and both sides are filled with small holes. Only in adult individuals has the front of the premaxillary contact been fused. Longer than the premaxilla, the nasal bones are the longest in the skull. They make up the entire length of the crest, and are never preserved as fused.